Fertilizers that improve the physical properties of the soil and, through these, influence the chemical and biological properties are amending. A material that balances an anomalous texture or that gives structure to an inconsistent terrain or that improves its structural state stabilizing it is therefore to be considered a soil conditioner. For example, the following materials are to be considered as amendments to the specific context:

  • clayey materials, limited to the contribution in sandy soils (balancing action on the weaving);
  • sand, limited to the contribution in clayey soils (balancing action on the weaving);
  • manure or other organic material that increases the level of humus in the soil (improving and stabilizing action on the structure).

The amendments to the amendments for the modification of the weaving require substantial amounts of material, of the order of hundreds of tons per hectare for each decimeter of depth. Even in the case of availability of low-cost material, the economic burden for transport, distribution and embedding is such that these interventions are not convenient and, in any case, sustainable only on small surfaces and for special purposes. A secondary amalgamation effect occurs with reclamation to fill when waters containing suspended material are used.
The amending interventions aimed at improving and stabilizing the structure require lower organic fertilizer inputs, of the order of tens of tons per hectare in case of extraordinary interventions or of a few tons per hectare in case of periodic interventions. The limited availability of organic soil improvers and the non-negligible costs of transport and distribution mean that these interventions are mostly carried out in specific contexts. In ordinary agricultural practice, by tradition, the supply of manure for soil improvers is carried out in the following cases:

  • as an extraordinary intervention before the planting of an orchard or a vineyard;
  • as a periodic intervention before the sowing of the cultivation by renewal in the context of a crop rotation;
  • as a systematic intervention in high-income crops (horticulture) on loose land.


They are corrective those fertilizers that improve the acidity of the soil by shifting the pH from abnormal values towards neutrality. Potentially they have corrective action all those materials that are constitutionally or physiologically acidic or alkaline. Lime and limestone in acid reaction soils, and sulfur and gypsum in basic soils with constitutional alkalinity (calcareous soils) are traditionally considered corrective. The effectiveness of the corrections is conditioned by various factors:

  •  Chemical behavior of the prevalent components. In this case the composition of the corrective and the chemical nature of its components are fundamental: the bases and the acids have a corrective action more effective than the salts, whose action is basically due to phenomena of saline hydrolysis or selective absorption by the plants (physiologically acidic or basic materials). For example, lime and sulfur have a more effective corrective action, respectively, of limestone and gypsum.
  • Degree of fineness of the material. The degree of fineness has reflections on the developing surface and is of particular importance for the corrective measures represented by poorly soluble salts such as limestone and gypsum. These materials, in fact, have a relative effectiveness, as corrective, only if finely ground, while the coarse grinding makes them substantially inert.
  • Buffering power of the soil. The soil exchange complex and, to a lesser extent, the composition of the mineral fraction of the circulating solution have an intrinsic buffering power which tends to compensate for changes in pH. The buffer power manifests itself more intensely precisely in relation to the corrective interventions, in any case it varies according to the causes that are the basis of the anomalous reaction.

The opportunity for corrective action is determined by real needs, economic sustainability and the intrinsic effectiveness of the intervention, especially in relation to the buffer power of the land. The competition of these factors makes it vary considerably according to the context:

  • Sodium lands. Soils with alkalinity of absorption, due to an excess of sodium in the exchange complex and sodium salts with alkaline hydrolysis in the circulating solution (sodium carbonate and sodium bicarbonate), present agronomic problems such as to render it inappropriate, if not all improper, the correction. The poor agronomic vocation is in fact due to the negative effects of sodium on the structure of the soil, which prevail over the interference, by the pH, with the mineral nutrition of the plants.
  • Calcareous soils. The soils with constitutional alkalinity, due to an excess of active limestone can be corrected with the addition of sulfur or gypsum, with effectiveness subordinated to the effective buffer power of the soil. The acidifying corrective drugs have a relatively high cost that can hardly be offset by the remunerativeness of the benefits of the correction. It should also be noted that calcareous soils can offer good productivity performance with an adequate choice of crops (agrarian species, cultivars, rootstocks) and with the adoption of appropriate agronomic measures.
  • Acid soils. In these soils the abnormal reaction is the main limiting factor as it negatively interferes with mineral nutrition, both due to the lack of bases (in particular calcium), and to the phosphorus insolubility. In the alternative to the buffer effect, acid soils are therefore those that respond best to pH correction.

For the above reasons, the opportunity of corrective interventions is evaluated almost exclusively for the correction of acid soils, also due to the more accessible cost of lime. The corrective quantities necessary for the intervention can be estimated by means of chemical analysis, with the determination of the requirement at the bottom. This determination, conducted in the laboratory, follows a standard procedure that differs more or less markedly from the actual operating conditions and, generally, the result in the laboratory underestimates the actual needs, mainly due to the complexity of the factors involved in determining the power soil buffer.
In general, the cost of correcting an acid medium is such that it does not allow significant variations in the reaction, however, the increase of a few tenths of a pH unit may allow an expansion of the range of cultivable species to the point of making it economically convenient. correction.


Fertilizers that improve the endowment of the land in one or more elements of fertility are fertilizers. These fertilizers are therefore made in order to increase the endowment of a poor soil (bottom fertilization) or to meet the nutritional needs of a crop without incurring the depletion of soil fertility (ordinary fertilization of production).

Fertilizers are the fertilizers most used in agriculture and their use is necessary above all in intensive agriculture, aimed at achieving high unit yields. In other conditions, the non-use of fertilization entails a slow but progressive impoverishment of the soil, which in more or less long periods undergo phenomena of intolerance. This problem is particularly felt in the tropical areas of the developing countries, where the cultivation of natural soils, in the absence of fertilization interventions, intensifies the process of desertification due to the modest absorbing power of the soil and the leaching of the nutritive elements in rainy climates.
The fertilizers are classified mainly according to the origin of the material and the chemical composition, with reference to the content in one or more main elements of fertility (nitrogen, phosphorus, potassium).

According to the first criterion, fertilizers are divided into three categories:

  • Organic fertilizers. They are materials of biological, animal or plant origin, used as they are or appropriately treated. Organic fertilizers, for example, manure, compost, pollen, etc.
  • Mineral or chemical fertilizers. They are materials obtained from a chemical synthesis in an industrial process (eg urea) or from extraction with possible industrial treatment (eg mineral superphosphate, potassium chloride).
  • Organomineral fertilizers. They are materials obtained from the mixing of organic fertilizers with chemical fertilizers or from the treatment of organic materials with chemical agents.

According to the second criterion, applied fundamentally to chemical fertilizers, fertilizers are classified as follows:

  • Ternary fertilizers: contain the three main macroelements of fertility (NPK).
  • Binary fertilizers: contain only two main macroelements of fertility. They are distinguished in nitrogen-potassium (NK), nitrogen-phosphates (NP) and phosphopotaxes (PK).
  • Simple fertilizers: they contain only one main macro-element of fertility, possibly associated with one or more secondary macro-elements (calcium, sulfur, magnesium) or with micro-nutrients (iron, copper, boron, etc.). They are distinguished in nitrogen, phosphates and potassic.

The quantities of fertilizer used with the fertilizer are significantly lower than those made for an ameliorating effect or for a correction of the pH, this fact which often makes this fertilization intervention profitable. Fertilizer doses vary depending on various factors, such as the nature and title of the fertilizer, the needs of the crops, the expected quantitative and qualitative yields, the environmental impact, the regulatory aspects, etc.
In the case of chemical fertilizers, the contributions are decidedly minimal, of a few quintals per hectare per year: excessively high doses have more or less serious negative effects on the production due to the increase in soil salinity, to any phytotoxicity, the onset of antagonistic phenomena between nutritive elements, interference with trophic relations between agrarian plant and parasites, etc. To these must be added any damage to the environment, mainly due to pollution of groundwater and potential health risks, related to the accumulation of nitrates in agricultural products.
In the case of organic fertilizers, provided with a much lower titer than chemical fertilizers, technically permissible doses are significantly higher and vary from a few tens of quintals per hectare per year to a few hundred. The factors limiting the quantities, in this case, are of an economic nature (excessive cost of transport and distribution), technical (low carbon-nitrogen ratio with consequent tendency to a rapid mineralization), environmental (risk of air pollution for the emanation of unpleasant odors and groundwater).